feat: add maintenance tree_type with db migration and tests

- Expand ck_trees_tree_type CHECK constraint to include 'maintenance'
- Add 'maintenance' to TreeType Literal in schemas
- Treat maintenance trees as procedural in can_publish_tree validation
- Alembic migration 0f1ca2af3647 drops and recreates the constraint
- Two integration tests: create and filter by tree_type=maintenance

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/alembic/versions/0f1ca2af3647_add_maintenance_tree_type.py

@@ -0,0 +1,33 @@
+"""add maintenance tree type
+
+Revision ID: 0f1ca2af3647
+Revises: 039
+Create Date: 2026-02-17 10:25:54.959861
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = '0f1ca2af3647'
+down_revision: Union[str, None] = '039'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.execute("ALTER TABLE trees DROP CONSTRAINT ck_trees_tree_type")
+    op.execute(
+        "ALTER TABLE trees ADD CONSTRAINT ck_trees_tree_type "
+        "CHECK (tree_type IN ('troubleshooting', 'procedural', 'maintenance'))"
+    )
+
+
+def downgrade() -> None:
+    op.execute("UPDATE trees SET tree_type = 'procedural' WHERE tree_type = 'maintenance'")
+    op.execute("ALTER TABLE trees DROP CONSTRAINT ck_trees_tree_type")
+    op.execute(
+        "ALTER TABLE trees ADD CONSTRAINT ck_trees_tree_type "
+        "CHECK (tree_type IN ('troubleshooting', 'procedural'))"
+    )